The acryl syrup has been used as an intermediate raw-material for a (meth)acryl resin substrate, optical materials such as an optical transmission fiber or an optical waveguide, artificial marbles, floor materials, adhesives and medical supplies.
The acryl syrup can be produced by bulk polymerization, solution polymerization, emulsion polymerization, suspension polymerization and photo polymerization. However, solution polymerization, emulsion polymerization and suspension polymerization have many disadvantages in that the methods require too much energy in order to remove reaction residues. In addition, it is difficult to obtain a product exhibiting high functionality, and the load to environment is high. For these reasons, it is a tendency to produce acryl syrup by bulk polymerization or photo polymerization, which is carried out in the absence of solvents.
However, in carrying out bulk polymerization or photo polymerization, heat transfer is difficult, since the polymerization is carried out without solvents. Furthermore, in case the polymerization is carried out in a general batch reactor, runaway reaction is easy to occur, since the reactor temperature is very difficult to control. Also, the termination of production of radicals is reduced due to a rapid increase in viscosity resulting from an increase in conversion rate. Thus, a phenomenon such as the partial formation of gel occurs, and non-uniform resin with wide range of molecular weight is liable to be obtained.
In order to overcome the above problems, the attempt to use a semi-batch, continuous or plug flow reactor in place of the batch reactor has been reported. Japanese patent laid-open publication Nos. S40-003701, H11-255828 and 2000-159816 disclose a method, in which polymerization is carried out under high temperature by using a continuous polymerization method.
However, the polymerization in those reactors has a problem in that it has a high economic burden, since not only the reactor itself but also the cost of utilities is expensive. Another problem is that it is unfavorable for the production of small amounts of many different species, although it is suitable for the production of large amounts of a few species.
In an attempt to solve these problems, a method is known that polymerization carries out in the batch reactor under the mild conditions. This is a method where the temperature of a reaction system is maintained at a constant level while forcibly terminating polymerization when the conversion or viscosity of the reaction system reaches a given level. Regarding the method for the termination of polymerization, Japanese patent laid-open publication No. H01-11652 discloses the termination of polymerization by the addition of a polymerization inhibitor, and Japanese patent laid-open publication No. H09-67495 discloses polymerization termination, such as quenching by the addition of monomers.
However, this polymerization method cannot become a fundamental solution, because it has shortcomings to cause a great increase in viscosity at the latter half of reaction, shows a difference in physical properties depending on the completion time of reaction, and results in polymer syrup with poor storage stability because a polymerization initiator remains in the syrup.
In an attempt to solve these problems, there have been many solutions where a batch reactor is used but runaway reaction does not occur and the molecular weight of syrup is easily controlled.
First, there are examples where bulk polymerization is carried out without using a polymerization initiator.
Japanese patent laid-open publication No. 2001-31709 discloses a method, in which the bulk polymerization is carried out by using a compound having a thiol group and a secondary hydroxyl group with a compound having a secondary hydroxyl group and not having a thiol group. Also, Japanese patent laid-open publication No. 2001-302705 discloses a method, in which the bulk polymerization is carried out by using a compound having a thiol group and a carboxyl group.
However, in the above polymerization methods, the residue of thiol groups in syrup deteriorates the physical properties of products, such as storage stability.
Japanese patent laid-open publication No. 2000-313704 discloses a method synthesizing acryl syrup having the conversion rate of 10-50%, in which 0.0001-0.5 parts by weight of an polymerization initiator having a 10-hr half-life at the temperature of less than 41° C. is used, the self-heat generation at reaction temperature of 20-80° C. is used, and the peak exothermic temperature of reactants reaches a range of 100-140° C. This polymerization method is based on the idea that the rapid increase in the concentration of radicals at the initial stage of reaction, resulted from the addition of an initiator which used to be decomposed at low temperature at a relatively high reaction temperature, causes the rapid increase of reaction speed, and as a result, the reactor temperature is increased, the half-life of the initiator drops rapidly and finally the depletion of initiator occurs resulting in stopping of reaction.
However, from the thorough examination, the present inventors concluded that this method needs a proper amount of an initiator to obtain a satisfactory result, which suggests a problem of high exothermic temperature. Thus, a highly sensitive cooling system is required during scale-up, nevertheless there is still a possibility of runaway reaction. In addition, the method which has the peak exothermic temperature of reactants ranging 100-140° C. is not applicable to low boiling point monomers such as ethyl acrylate (boiling point=99° C.) and butyl acrylate (boiling point=145° C.). Also, there is a problem of rapid spontaneous thermo polymerization by a high temperature, unless the reactor is not rapidly cooled down at such high temperature. In the meantime, in production of high molecular weight acryl syrup having more than a million of molecular weight, the conversion rate should be maintained as low as possible, because of the steep increase in viscosity, but the method described in Japanese patent laid-open publication No. 2000-313704 is carried out without lowering the conversion rate, meaning that it has problems of unstable stirring and formation of gel due to the rapid increase of viscosity to produce a syrup having more than a million of molecular weight.